1. Field of the Invention
The present invention relates to FM modulators, and more specifically to an FM modulator for generating a wide-band frequency-modulated signal (hereinafter referred to as an FM signal) using a semiconductor laser and optical heterodyne detection.
2. Description of the Background Art
FIG. 7 is a block diagram showing the structure of a conventional FM modulator. The FM modulator with this structure is shown, for example, in a reference (K. Kikushima, et al, xe2x80x9cOptical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systemsxe2x80x9d, IOOC ""95 Technical Digest, Vol. 5, PD2-7, pp.33-34). In FIG. 7, the FM modulator includes a signal source 600, a driving amplifier 602, a frequency modulation laser (hereinafter referred to as an FM laser) 604, a local light source 605, and an optical-electrical converting portion 606.
In the above structured FM modulator, the signal source 600 outputs an electrical signal which is an original signal for FM modulation and the driving amplifier 602 amplifies the electrical signal. The FM laser 604, which is structured of a semiconductor laser element and the like, for example, oscillates light having a wavelength xcex1 on condition that an injection current is constant. When the injection current is amplitude-modulated, the outputted light is modulated in an oscillation wavelength (optical frequency) as well as in intensity, and the FM laser 604 outputs an optical frequency-modulated signal having the wavelength xcex1 at the center. The local light source 605 outputs unmodulated light having a wavelength xcex0 which is different from the oscillation wavelength xcex1 of the FM laser 604 by a prescribed amount xcex94xcex1. The outputted optical signal from the FM laser 604 and the outputted light from the local light source 605 are combined to be inputted to the optical-electrical converting portion 606. The optical-electrical converting portion 606 is structured of a photodiode having a square-law detection characteristic, and the like, and generally has the properties of converting an optical intensity modulation component of the inputted light into a current amplitude modulation component (hereinafter referred to as an optical intensity modulation/direct detection component: an IM-DD component) and, when two lightwaves having different wavelengths are inputted, generating a beat component of the two lightwaves at a frequency corresponding to the wavelength difference (this operation is called an optical heterodyne detection). Accordingly, the optical-electrical converting portion 606 outputs the beat signal of the outputted optical signal from the FM laser 604 and the outputted light from the local light source 605 at a frequency corresponding to the wavelength difference xcex94xcex between the two lightwaves.
The beat signal obtained as described above is an FM signal taking the electrical signal from the signal source 600 as an original signal. Therefore, by using the appropriate FM laser 604 and local light source 605, it is possible to easily generate a high-frequency and wide-band FM signal having a center frequency (carrier frequency) more than several GHz and frequency deviation more than several hundred MHZ, which it is difficult to realize in an FM modulator with an ordinary electric circuit.
In the conventional FM modulator having the above structure, a carrier-to-noise ratio (hereinafter referred to as a CNR), which shows the quality of the FM signal, is improved as the frequency deviation in the FM laser 604 increases and as spectral line widths of the FM laser 604 and the local light source 605 become narrower. The spectral line widths of these two light sources are parameters depending on the composition and structure of each light source and cannot be changed greatly by limitations such as use conditions and the like. However, if the amplitude of the inputted signal to the FM laser 604 is increased, it is possible to increase the frequency deviation and thus improve the CNR. However, since the laser light source has a threshold characteristic, when the amplitude of the inputted signal is increased to more than a prescribed degree, a distortion characteristic is extremely deteriorated due to clipping of the signal amplitude and the like. Furthermore, the outputted signal level of the driving amplifier 602 has a limit (saturated level), and if the outputted signal level is increased over a prescribed level, the distortion characteristic is sharply deteriorated. Therefore, the amplitude increase of the inputted signal to the FM laser 604 is limited, and it is thus disadvantageously difficult to improve the CNR to more than a prescribed degree.
Therefore, an object of the present invention is to provide an FM modulator capable of further improving a CNR.
In order to achieve the above object, the present invention has the following feature.
A first aspect of the present invention is to an FM modulator for converting an electrical signal into a frequency-modulated signal by an optical heterodyne method, comprising:
a branch portion for outputting, when the electrical signal is inputted, a phase-uninverted signal (hereinafter referred to as an in-phase signal) and a phase-inverted signal (hereinafter referred to as an opposite phase signal);
a first frequency modulation light source element (hereinafter referred to as a first FM light source element) having a property of uniquely converting an amplitude change in the inputted electrical signal into an optical frequency change of outputted light, for converting the in-phase signal into a frequency-modulated first optical signal having a center wavelength xcex1;
a second frequency modulation light source element (hereinafter referred to as a second FM light source element) having a property of uniquely converting an amplitude change in the inputted electrical signal into an optical frequency change of outputted light, for converting the opposite phase signal into a frequency-modulated second optical signal having a center wavelength xcex2; and
an optical-electrical converting portion for subjecting the first and second optical signals to optical heterodyne detection and then generating a beat signal at a frequency corresponding to a wavelength difference xcex94xcex(=|xcex1xe2x88x92xcex2|) between both of the optical signals.
As described above, in accordance with the first aspect, since the first and second FM light source elements perform modulating operation by electrical signals having an opposite phase relationship with each other, polarities of the frequency deviation in outputted lightwaves (the first and second optical signals) from the first and second FM light source elements also have an opposite phase relationship with each other. That is, when the first optical signal is deviated to a high frequency side, the second optical signal is deviated to a low frequency side, and on the contrary, when the first optical signal is deviated to a low frequency side, the second optical signal is deviated to a high frequency side. Therefore, in the optical-electrical converting portion, the frequency deviation of a beat signal obtained as a difference signal between these two optical signals is the sum of the frequency deviation of the first optical signal and the frequency deviation of the second optical signal. Therefore, compared to the conventional FM modulator, the frequency deviation of the outputted signal is increased, allowing great improvement in a CNR performance.
According to a second aspect of the present invention, in the first aspect, the FM modulator further comprises an amplitude adjusting portion inserted at least either between the branch portion and the first FM light source element or between the branch portion and the second FM light source element, for adjusting amplitude of the inputted electrical signal to equate frequency deviation of the first and second optical signals.
As described above, in accordance with the second aspect, the amplitudes of the electrical signal inputted into the first and second FM light source elements are appropriately adjusted to equate the frequency deviation in the first and second optical signals. Thus, also when light source elements whose amounts of wavelength chirping (a frequency change ratio with respect to inputted current amplitude) are different are used as the first and second FM light source elements, the frequency deviations of the optical signals are equal. As a result, in the optical-electrical converting portion, the frequency deviation of a beat signal obtained as a difference signal between these two optical signals is increased twice compared to that in the conventional FM modulator. Further, by making the frequency changes in the first and second FM light source elements symmetrical, it is possible to reduce a distortion generated in each of the first and second FM light source elements in current/optical frequency converting operation
According to a third aspect of the present invention, in the first aspect, the FM modulator further comprises a delay adjusting portion inserted on at least either a first route from the branch portion through the first FM light source element to the optical-electrical converting portion or a second route from the branch portion through the second FM light source element to the optical-electrical converting portion, for adjusting propagation delay of a signal propagated on the route to equate propagation delay included in the first route and propagation delay included in the second route.
In the above third aspect, by adjusting propagation delay at an appropriate position on two propagation routes from the branch portion to the optical-electrical converting portion to equate propagation time on both propagation routes, it is possible to more ideally generate a wide-band FM signal with its frequency deviation expanded.
According to a fourth aspect, in the first aspect, the FM modulator further comprises a level adjusting portion inserted on at least either a first route from the branch portion through the first FM light source element to the optical-electrical converting portion or a second route from the branch portion through the second FM light source element to the optical-electrical converting portion, for adjusting signal power to equate an amplitude of an optical intensity modulation/direct detection component outputted by subjecting an optical intensity modulation component included in the first optical signal to square-law detection in the optical-electrical converting portion and an amplitude of an optical intensity modulation/direct detection component outputted by subjecting an optical intensity modulation component included in the second optical signal to square-law detection in the optical-electrical converting portion.
Since a semiconductor laser can easily generate an optical frequency-modulated signal by modulating its injection current, it is suitable for the first and second FM light element. However, when the injection current to the semiconductor laser is subjected to modulation, the outputted light is subjected to modulation in intensity, and when the resultant light is subjected to square-law detection in the optical-electrical converting portion, an optical intensity modulation/direct detection component (IM-DD component) is generated. The IM-DD component is an unwanted signal with respect to the wide-band FM signal, deteriorating the quality of the FM demodulation signal.
When the first and second FM light sources perform modulating operation by electrical signals whose phases are opposite, the first and second IIM-DD components also have an opposite phase relationship with each other. Therefore, in the above fourth aspect, signal power is adjusted at appropriate positions on two propagation routes from the branch portion to the optical-electrical converting portion to equate the magnitudes of the first and second IM-DD components, and these components are canceled/suppressed in the optical-electrical converting portion. It is thus possible to generate a wide-band FM signal with high quality without an unwanted signal.
A fifth aspect of the present invention is an FM modulator for converting an electrical signal into a frequency-modulated signal by an optical heterodyne method, comprising:
a branch portion for outputting, when the electrical signal is inputted, a phase-uninverted signal (hereinafter referred to as an in-phase signal) and a phase-inverted signal (hereinafter referred to as an opposite phase signal);
a first light source for outputting unmodulated light with a wavelength xcex1;
a first optical phase modulating portion having a property of uniquely converting an amplitude change in the inputted electrical signal into an optical phase change of outputted light, for converting, when outputted light from the first light source is inputted, the in-phase signal into a phase-modulated first optical signal having a center frequency xcex1;
a second light source for outputting unmodulated light with a wavelength xcex2;
a second optical phase modulating portion having a property of uniquely converting an amplitude change in the outputted electrical signal into an optical phase change of inputted light, for converting, when outputted light from the second light source is inputted, the opposite phase signal into a phase-modulated second optical signal having a center frequency xcex2; and
an optical-electrical converting portion for subjecting the first and second optical signals to optical heterodyne detection and then generating a beat signal at a frequency corresponding to a wavelength difference xcex94xcex(=|xcex1xe2x88x92xcex2|) between both optical signals.
In the above described first to fourth aspects, as a method for producing an optical frequency-modulated signal, a structure is adopted in which a wavelength chirping characteristic of the FM light source element (for example, a semiconductor laser) is used and subjected to direct modulation. Another structure can be thought in which an external optical phase modulator is used in order to produce an optical frequency-modulated signal. As is generally well known, frequency modulation and phase modulation can be thought substantially the same.
Therefore, in the fifth aspect, in place of the first and second FM light source elements, the first and second optical phase modulating portions are provided with unmodulated light from the first and second light sources, respectively, and then subject the light to optical phase modulation. Since the first and second optical phase modulating portions perform modulating operation by electrical signals whose phases are opposite, polarities of frequency deviation in their outputted lights also have an opposite phase relationship with each other. Therefore, as in the fifth aspect, the frequency deviation of the beat signal obtained as a difference signal between these two optical signals is the sum of the frequency deviation of the outputted light from the first optical phase modulating portion and the frequency deviation of the outputted light from the second optical phase modulating portion. Thus, the frequency deviation of the outputted signal is increased compared to the conventional FM modulator, allowing great improvement in a CNR performance.
According to a sixth aspect of the present invention, in the fifth aspect, the FM modulator further comprises an amplitude adjusting portion inserted at least either between the branch portion and the first optical phase modulation portion or between the branch portion and the second optical phase modulation portion, for adjusting amplitude of the inputted electrical signal to equate frequency deviation of the first and second optical signals.
As described above, according to the sixth aspect, the amplitudes of the electrical signals inputted into the first and second optical phase modulation portions are appropriately adjusted to equate the frequency deviation in the first and second optical signals. Thus, also when external optical phase modulators whose optical phase modulation efficiencies are different are used as the first and second optical phase modulating portions, the frequency deviations of the optical signals are equal. As a result, in the optical-electrical converting portion, the frequency deviation of a beat signal obtained as a difference signal between these two optical signals is increased twice compared to that in the conventional FM modulator. Further, by making the frequency changes in the first and second optical phase modulating portions symmetrical, it is possible to reduce a distortion generated in each of the first and second optical phase modulating portions in voltage/optical frequency converting operation.
According to a seventh aspect of the present invention, in the fifth aspect, the FM modulator further comprises a delay adjusting portion inserted on at least either a first route from the branch portion through the first optical phase modulation portion to the optical-electrical converting portion or a second route from the branch portion through the second optical phase modulation portion to the optical-electrical converting portion, for adjusting propagation delay of a signal propagated on the routes to equate propagation delay included in the first route and propagation delay included in the second route.
In the above seventh aspect, by adjusting propagation delay at an appropriate position on two propagation routes from the branch portion to the optical-electrical converting portion to equate propagation time on both propagation routes, it is possible to more ideally generate a wide-band FM signal with its frequency deviation expanded.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.